.TH  CGBRFSX 1 "November 2008" "    LAPACK routine (version 3.2)                                 " "    LAPACK routine (version 3.2)                                 " 
.SH NAME
CGBRFSX - CGBRFSX improve the computed solution to a system of linear  equations and provides error bounds and backward error estimates  for the solution
.SH SYNOPSIS
.TP 20
SUBROUTINE CGBRFSX(
TRANS, EQUED, N, KL, KU, NRHS, AB, LDAB, AFB,
LDAFB, IPIV, R, C, B, LDB, X, LDX, RCOND,
BERR, N_ERR_BNDS, ERR_BNDS_NORM,
ERR_BNDS_COMP, NPARAMS, PARAMS, WORK, RWORK,
INFO )
.TP 20
.ti +4
IMPLICIT
NONE
.TP 20
.ti +4
CHARACTER
TRANS, EQUED
.TP 20
.ti +4
INTEGER
INFO, LDAB, LDAFB, LDB, LDX, N, KL, KU, NRHS,
NPARAMS, N_ERR_BNDS
.TP 20
.ti +4
REAL
RCOND
.TP 20
.ti +4
INTEGER
IPIV( * )
.TP 20
.ti +4
COMPLEX
AB( LDAB, * ), AFB( LDAFB, * ), B( LDB, * ),
X( LDX , * ),WORK( * )
.TP 20
.ti +4
REAL
R( * ), C( * ), PARAMS( * ), BERR( * ),
ERR_BNDS_NORM( NRHS, * ),
ERR_BNDS_COMP( NRHS, * ), RWORK( * )
.SH PURPOSE
   CGBRFSX improves the computed solution to a system of linear
   equations and provides error bounds and backward error estimates
   for the solution.  In addition to normwise error bound, the code
   provides maximum componentwise error bound if possible.  See
   comments for ERR_BNDS_N and ERR_BNDS_C for details of the error
   bounds.
.br
   The original system of linear equations may have been equilibrated
   before calling this routine, as described by arguments EQUED, R
   and C below. In this case, the solution and error bounds returned
   are for the original unequilibrated system.
.br
.SH ARGUMENTS
Some optional parameters are bundled in the PARAMS array.  These
settings determine how refinement is performed, but often the
defaults are acceptable.  If the defaults are acceptable, users
can pass NPARAMS = 0 which prevents the source code from accessing
the PARAMS argument.
.TP 8
TRANS   (input) CHARACTER*1
Specifies the form of the system of equations:
.br
= \(aqN\(aq:  A * X = B     (No transpose)
.br
= \(aqT\(aq:  A**T * X = B  (Transpose)
.br
= \(aqC\(aq:  A**H * X = B  (Conjugate transpose = Transpose)
.TP 8
EQUED   (input) CHARACTER*1
Specifies the form of equilibration that was done to A
before calling this routine. This is needed to compute
the solution and error bounds correctly.
= \(aqN\(aq:  No equilibration
.br
= \(aqR\(aq:  Row equilibration, i.e., A has been premultiplied by
diag(R).
= \(aqC\(aq:  Column equilibration, i.e., A has been postmultiplied
by diag(C).
= \(aqB\(aq:  Both row and column equilibration, i.e., A has been
replaced by diag(R) * A * diag(C).
The right hand side B has been changed accordingly.
.TP 8
N       (input) INTEGER
The order of the matrix A.  N >= 0.
.TP 8
KL      (input) INTEGER
The number of subdiagonals within the band of A.  KL >= 0.
.TP 8
KU      (input) INTEGER
The number of superdiagonals within the band of A.  KU >= 0.
.TP 8
NRHS    (input) INTEGER
The number of right hand sides, i.e., the number of columns
of the matrices B and X.  NRHS >= 0.
.TP 8
AB      (input) DOUBLE PRECISION array, dimension (LDAB,N)
The original band matrix A, stored in rows 1 to KL+KU+1.
The j-th column of A is stored in the j-th column of the
array AB as follows:
AB(ku+1+i-j,j) = A(i,j) for max(1,j-ku)<=i<=min(n,j+kl).
.TP 8
LDAB    (input) INTEGER
The leading dimension of the array AB.  LDAB >= KL+KU+1.
.TP 8
AFB     (input) DOUBLE PRECISION array, dimension (LDAFB,N)
Details of the LU factorization of the band matrix A, as
computed by DGBTRF.  U is stored as an upper triangular band
matrix with KL+KU superdiagonals in rows 1 to KL+KU+1, and
the multipliers used during the factorization are stored in
rows KL+KU+2 to 2*KL+KU+1.
.TP 8
LDAFB   (input) INTEGER
The leading dimension of the array AFB.  LDAFB >= 2*KL*KU+1.
.TP 8
IPIV    (input) INTEGER array, dimension (N)
The pivot indices from SGETRF; for 1<=i<=N, row i of the
matrix was interchanged with row IPIV(i).
.TP 8
R       (input or output) REAL array, dimension (N)
The row scale factors for A.  If EQUED = \(aqR\(aq or \(aqB\(aq, A is
multiplied on the left by diag(R); if EQUED = \(aqN\(aq or \(aqC\(aq, R
is not accessed.  R is an input argument if FACT = \(aqF\(aq;
otherwise, R is an output argument.  If FACT = \(aqF\(aq and
EQUED = \(aqR\(aq or \(aqB\(aq, each element of R must be positive.
If R is output, each element of R is a power of the radix.
If R is input, each element of R should be a power of the radix
to ensure a reliable solution and error estimates. Scaling by
powers of the radix does not cause rounding errors unless the
result underflows or overflows. Rounding errors during scaling
lead to refining with a matrix that is not equivalent to the
input matrix, producing error estimates that may not be
reliable.
.TP 8
C       (input or output) REAL array, dimension (N)
The column scale factors for A.  If EQUED = \(aqC\(aq or \(aqB\(aq, A is
multiplied on the right by diag(C); if EQUED = \(aqN\(aq or \(aqR\(aq, C
is not accessed.  C is an input argument if FACT = \(aqF\(aq;
otherwise, C is an output argument.  If FACT = \(aqF\(aq and
EQUED = \(aqC\(aq or \(aqB\(aq, each element of C must be positive.
If C is output, each element of C is a power of the radix.
If C is input, each element of C should be a power of the radix
to ensure a reliable solution and error estimates. Scaling by
powers of the radix does not cause rounding errors unless the
result underflows or overflows. Rounding errors during scaling
lead to refining with a matrix that is not equivalent to the
input matrix, producing error estimates that may not be
reliable.
.TP 8
B       (input) REAL array, dimension (LDB,NRHS)
The right hand side matrix B.
.TP 8
LDB     (input) INTEGER
The leading dimension of the array B.  LDB >= max(1,N).
.TP 8
X       (input/output) REAL array, dimension (LDX,NRHS)
On entry, the solution matrix X, as computed by SGETRS.
On exit, the improved solution matrix X.
.TP 8
LDX     (input) INTEGER
The leading dimension of the array X.  LDX >= max(1,N).
.TP 8
RCOND   (output) REAL
Reciprocal scaled condition number.  This is an estimate of the
reciprocal Skeel condition number of the matrix A after
equilibration (if done).  If this is less than the machine
precision (in particular, if it is zero), the matrix is singular
to working precision.  Note that the error may still be small even
if this number is very small and the matrix appears ill-
conditioned.
.TP 8
BERR    (output) REAL array, dimension (NRHS)
Componentwise relative backward error.  This is the
componentwise relative backward error of each solution vector X(j)
(i.e., the smallest relative change in any element of A or B that
makes X(j) an exact solution).
N_ERR_BNDS (input) INTEGER
Number of error bounds to return for each right hand side
and each type (normwise or componentwise).  See ERR_BNDS_NORM and
ERR_BNDS_COMP below.
.TP 15
ERR_BNDS_NORM  (output) REAL array, dimension (NRHS, N_ERR_BNDS)
For each right-hand side, this array contains information about
various error bounds and condition numbers corresponding to the
normwise relative error, which is defined as follows:
Normwise relative error in the ith solution vector:
max_j (abs(XTRUE(j,i) - X(j,i)))
------------------------------
max_j abs(X(j,i))
The array is indexed by the type of error information as described
below. There currently are up to three pieces of information
returned.
The first index in ERR_BNDS_NORM(i,:) corresponds to the ith
right-hand side.
The second index in ERR_BNDS_NORM(:,err) contains the following
three fields:
err = 1 "Trust/don\(aqt trust" boolean. Trust the answer if the
reciprocal condition number is less than the threshold
sqrt(n) * slamch(\(aqEpsilon\(aq).
err = 2 "Guaranteed" error bound: The estimated forward error,
almost certainly within a factor of 10 of the true error
so long as the next entry is greater than the threshold
sqrt(n) * slamch(\(aqEpsilon\(aq). This error bound should only
be trusted if the previous boolean is true.
err = 3  Reciprocal condition number: Estimated normwise
reciprocal condition number.  Compared with the threshold
sqrt(n) * slamch(\(aqEpsilon\(aq) to determine if the error
estimate is "guaranteed". These reciprocal condition
numbers are 1 / (norm(Z^{-1},inf) * norm(Z,inf)) for some
appropriately scaled matrix Z.
Let Z = S*A, where S scales each row by a power of the
radix so all absolute row sums of Z are approximately 1.
See Lapack Working Note 165 for further details and extra
cautions.
.TP 15
ERR_BNDS_COMP  (output) REAL array, dimension (NRHS, N_ERR_BNDS)
For each right-hand side, this array contains information about
various error bounds and condition numbers corresponding to the
componentwise relative error, which is defined as follows:
Componentwise relative error in the ith solution vector:
abs(XTRUE(j,i) - X(j,i))
max_j ----------------------
abs(X(j,i))
The array is indexed by the right-hand side i (on which the
componentwise relative error depends), and the type of error
information as described below. There currently are up to three
pieces of information returned for each right-hand side. If
componentwise accuracy is not requested (PARAMS(3) = 0.0), then
ERR_BNDS_COMP is not accessed.  If N_ERR_BNDS .LT. 3, then at most
the first (:,N_ERR_BNDS) entries are returned.
The first index in ERR_BNDS_COMP(i,:) corresponds to the ith
right-hand side.
The second index in ERR_BNDS_COMP(:,err) contains the following
three fields:
err = 1 "Trust/don\(aqt trust" boolean. Trust the answer if the
reciprocal condition number is less than the threshold
sqrt(n) * slamch(\(aqEpsilon\(aq).
err = 2 "Guaranteed" error bound: The estimated forward error,
almost certainly within a factor of 10 of the true error
so long as the next entry is greater than the threshold
sqrt(n) * slamch(\(aqEpsilon\(aq). This error bound should only
be trusted if the previous boolean is true.
err = 3  Reciprocal condition number: Estimated componentwise
reciprocal condition number.  Compared with the threshold
sqrt(n) * slamch(\(aqEpsilon\(aq) to determine if the error
estimate is "guaranteed". These reciprocal condition
numbers are 1 / (norm(Z^{-1},inf) * norm(Z,inf)) for some
appropriately scaled matrix Z.
Let Z = S*(A*diag(x)), where x is the solution for the
current right-hand side and S scales each row of
A*diag(x) by a power of the radix so all absolute row
sums of Z are approximately 1.
See Lapack Working Note 165 for further details and extra
cautions.
NPARAMS (input) INTEGER
Specifies the number of parameters set in PARAMS.  If .LE. 0, the
PARAMS array is never referenced and default values are used.
.TP 8
PARAMS  (input / output) REAL array, dimension NPARAMS
Specifies algorithm parameters.  If an entry is .LT. 0.0, then
that entry will be filled with default value used for that
parameter.  Only positions up to NPARAMS are accessed; defaults
are used for higher-numbered parameters.
PARAMS(LA_LINRX_ITREF_I = 1) : Whether to perform iterative
refinement or not.
Default: 1.0
.br
= 0.0 : No refinement is performed, and no error bounds are
computed.
= 1.0 : Use the double-precision refinement algorithm,
possibly with doubled-single computations if the
compilation environment does not support DOUBLE
PRECISION.
(other values are reserved for future use)
PARAMS(LA_LINRX_ITHRESH_I = 2) : Maximum number of residual
computations allowed for refinement.
Default: 10
.br
Aggressive: Set to 100 to permit convergence using approximate
factorizations or factorizations other than LU. If
the factorization uses a technique other than
Gaussian elimination, the guarantees in
err_bnds_norm and err_bnds_comp may no longer be
trustworthy.
PARAMS(LA_LINRX_CWISE_I = 3) : Flag determining if the code
will attempt to find a solution with small componentwise
relative error in the double-precision algorithm.  Positive
is true, 0.0 is false.
Default: 1.0 (attempt componentwise convergence)
.TP 8
WORK    (workspace) REAL array, dimension (4*N)
.TP 8
IWORK   (workspace) INTEGER array, dimension (N)
.TP 8
INFO    (output) INTEGER
.br
= 0:  Successful exit. The solution to every right-hand side is
guaranteed.
< 0:  If INFO = -i, the i-th argument had an illegal value
.br
> 0 and <= N:  U(INFO,INFO) is exactly zero.  The factorization
has been completed, but the factor U is exactly singular, so
the solution and error bounds could not be computed. RCOND = 0
is returned.
= N+J: The solution corresponding to the Jth right-hand side is
not guaranteed. The solutions corresponding to other right-
hand sides K with K > J may not be guaranteed as well, but
only the first such right-hand side is reported. If a small
componentwise error is not requested (PARAMS(3) = 0.0) then
the Jth right-hand side is the first with a normwise error
bound that is not guaranteed (the smallest J such
that ERR_BNDS_NORM(J,1) = 0.0). By default (PARAMS(3) = 1.0)
the Jth right-hand side is the first with either a normwise or
componentwise error bound that is not guaranteed (the smallest
J such that either ERR_BNDS_NORM(J,1) = 0.0 or
ERR_BNDS_COMP(J,1) = 0.0). See the definition of
ERR_BNDS_NORM(:,1) and ERR_BNDS_COMP(:,1). To get information
about all of the right-hand sides check ERR_BNDS_NORM or
ERR_BNDS_COMP.
